Seismic sources are generally devices that generate acoustic energy. One use of seismic sources is in marine seismic surveying in which the seismic sources may be employed to generate acoustic energy that travels downwardly through water and into subsurface rock. After interacting with the subsurface rock, e.g., at boundaries between different subsurface layers, some of the acoustic energy may be returned toward the water surface and detected by specialized sensors (e.g., hydrophones, geophones, etc.). The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the seismic sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive-type sources are air guns that typically utilize compressed air to generate acoustic energy. Other examples of impulsive-type sources include explosives and weight-drop impulse sources. The frequency content of these impulsive-type sound sources typically in use today is controllable only to a small degree. As a result, different sound sources may be selected for the generation of different frequency ranges based on the surveying needs. Notably, these impulsive-type sources also have limited acoustic energy output in the very low frequency band from 1-10 Hz. However, it is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves may be attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. Thus, efforts have been undertaken to develop sound sources that can operate at low frequencies.